The invention is in the field of spectrum analyzers and specifically spectrum analyzers which have provisions for changing the frequency characteristics of the input signal such that it can be analyzed with greater resolution or more effectively than with more conventional spectrum analyzers. A fully digital spectrum analyzer of a type to which the invention pertains is disclosed in Lehmann et al., U.S. Pat. No. 3,881,097, which is hereby incorporated by reference here, and this invention is an improvement over spectrum analyzers of that type.
In the patented Lehmann et al. spectrum analyzer, and in other similar analyzers, an input signal is analyzed by finding its power spectrum at each of a given number of spectral lines which are uniformly distributed in the frequency range from zero to the maximum frequency of interest. For example, if the maximum frequency of interest in the input signal is 25.6 KHz and the spectrum analyzer has 256 spectral lines, the spectral lines are spaced by 100 Hz intervals, i.e., the resolution of the spectrum analyzer is 100 Hz. If the entire band from zero Hz to a given upper limit is of interest, then the 256 spectral lines are distributed optimally; however, if only a band starting above zero Hz is of interest, say if only the band from 12.2 KHz to 13.2 KHz is of interest, then that band is covered by only 10 spectral lines while the results for the remaining 246 available spectral lines are not used. It is desirable therefore to shift such band of interest down on the frequency scale so that it would start from zero Hz and the spectral lines can be efficiently distributed only or primarily within the band of interest.
It is known in the prior art to shift a frequency band that starts and ends at a non-zero frequency to a shifted band that starts at zero frequency. For example, Lathi, B.P., Communication Systems, Wiley, 1968, discusses at pages 179-184 an analog technique of generating a single sideband which has the effect of shifting a frequency band that starts at a non-zero frequency to a shifted band starting at zero frequency. A similar analog technique is used to shift a frequency band prior to applying it to a spectrum analyzer in Murtin, U.S. Pat. No. 3,634,760. It is also known to use certain digital techniques for effecting similar frequency shifts, and a digital technique relying on shifting which is matched to fast Fourier transform analysis is believed used in a spectrum analyzer sold by Sanders Associates, Inc. under the name S.A.-240. A technique which may have a similar effect of generating a single sideband, but which used processing based on Hilbert transforms, is disclosed in White, U.S. Pat. No. 3,800,131.
The analog techniques for shifting a frequency band described above may not be sufficiently accurate for a spectrum analyzer having certain minimum accuracy requirements, and it is desirable therefore to use digital techniques. However, the digital techniques for shifting a frequency band described above are matched to a spectrum analyzer using fast Fourier, Hilbert or similar transforms, and are not suited for other spectrum analyzers, such as the one disclosed in said Lehmann et al. patent, which use a discrete Fourier transform technique differing from the fast Fourier or Hilbert transform techniques in some respects which are important to the task of frequency band shifting. For example, while fast Fourier transform processors can receive the complex input (having real and imaginary parts) which results from the prior art shifting described above, a discrete Fourier transform processor of the type used in said Lehamnn et al. patent is suitable for only a real input. Accordingly, one aspect of the invention is to provide a spectrum analyzer in which a frequency band is shifted or translated by digital techniques which can work with a discrete Fourier transform processor rather than only with fast Fourier or similar transform processors.
The prior art spectrum analyzers referred to above are believed to do the frequency translation by processing circuits dedicated to that task, and to do it before the signal reaches the spectrum analyzer proper. In contrast, the invented analyzer does the frequency shift or translation to a substantial degree in the spectrum analyzer and by components which are time-shared for the tasks of frequency translation and conventional spectrum analysis, thus reducing the cost of providing for frequency translation capabilities.
Another type of modifying the frequency characteristics of a signal to be analyzed is used when the signal extends over a frequency range greater than the upper frequency limit for which the spectrum analyzer is designed. For example, if the spectrum analyzer is designed such that the maximum input frequency it can accept is 25.6 KHz, it can not accurately analyze signals which have a frequency of interest in excess of that limit. One technique for accommodating such fast input signal is to time-stretch the input signal, for example by sampling the fast input signal and deriving a time-stretched analog version of the sampled signal for feeding to the spectrum analyzer, as disclosed in detail in U.S. Pat. No. 3,969,705 issued on July 13, 1976 in the name of William N. Waggener and assigned to the same assignee as the invention here. The technique of the Waggener patent involves the use of a storage device, such as a charge transfer device serving as an analog delay line, and the use of circuitry for reading samples into the storage device at a high rate and reading the samples out of the storage device and into the spectrum analyzer at a low rate. While the technique is useful, the invention here has a corresponding provision for analyzing fast analog signals by changing them to a lower frequency content by a different, primarily analog technique.
In an exemplary embodiment of the invention, the spectrum analyzer can work in a first mode which is as described in said Lehmann et al patent. In this first mode, the analyzer converts an input analog signal to digital samples, multiplies the digital samples by selected trigonometric coefficients in a discrete Fourier transform (DFT) processor so as to generate DFT representations of the sampled signal, and then uses the discrete Fourier transform representations to calculate the power values for a set of spectral lines. Additionally, when it is desired to translate the frequency band of interest of the input signal, the spectrum analyzer operates in a second mode and uses much of the same discrete Fourier transform processor, on time-shared basis, to obtain discrete Fourier transform representations which correspond to a frequency band of the same width as the original frequency band of interest but translated to a zero center frequency. Still additionally, when it is desired to analyze a fast signal whose upper frequency limit is above the normal frequency limit of the spectrum analyzer, the spectrum analyzer works in a third mode to shift the frequencies of interest in the fast signal to lower frequencies, by primarily analog techniques, so that the originally fast signal can be effectively analyzed within the normal range of the spectrum analyzer.
More specifically, the spectrum analyzer provides a sequence of digital words, f(n), where each word is the n-th sample of N successive samples of the analog signal which is being analyzed (n = 1, 2, ..., N). In the second mode, the digital words, f(n), are multiplied by selected trigonometric functions to provide corresponding sequences of digital words, g(n) and h(n), resulting respectively from combining the word f(n) with corresponding first trigonometric functions of arguments including a value representing the center frequency of the frequency band of interest and corresponding second trigonometric functions of arguments including a value representing the same center frequency. The first and second trigonometric functions are in quadrature relationship with each other. The sequences of words g(n) and h(n) are filtered digitally to provide a sequence of words g(m) and a sequence of words h(m), where m = 1, 2, ..., M and M &lt; N. As one example, N = 25,600 ( = 50 .times. 512) and M = 512. The spectrum analyzer includes a discrete Fourier transform processor which receives the words g(m) and h(m) and processes them to provide for each spectral line k of K/2 spectral lines (where k = 1, 2, ..., K/2, and K is, for example, 256), a word RG and a word IG, where the RG word is the real part of the discrete Fourier transform of the words g(m) and the IG word is the imaginary part of the discrete Fourier transform of the same words g(m) for the spectral line k. The discrete Fourier transform processor further provides, for each spectral line k, a word RH and a word IH, where RH and IH are respectively the real and imaginary parts of the discrete Fourier transforms of the words h(m) for the spectral line k. For each spectral line k, the corresponding RG and IH words are combined to produce a word RU, and the corresponding IG and RH words are combined to produce a word IU, where the words RU and IU are respectively the real and imaginary parts of the upper sideband of the selected frequency band translated about zero. Additionally, for each of the spectral lines k the corresponding RG and IH words are combined to produce a word RL and the corresponding RH and IG words are combined to produce a word IL, where the RL and IL words are the real and imaginary parts respectively for the lower sideband of the selected center frequency translated about zero. The real and imaginary parts of the upper and lower sidebands are then processed by the power spectrum computing part of the spectrum analyzer, in the manner described in said Lehmann et al. patent, to get the power values for the spectral lines and to do any averaging that may be desired. The translation is done to a great extent by time-sharing the circuitry otherwise used for conventional spectrum analysis. A separate circuit, primarily analog, is used in the third mode to reduce the frequency content of very fast input signals so they can be effectively analyzed.